In copending U.S. Application Ser. No. 481,433 and now U.S. Pat. No. 3,911,681, incorporated herein by reference, it is disclosed how a stepped diameter piston can sequentially produce a first pressure force and a second pressure force when a transfer control valve is operated by movement of a plunger connected to an input push rod. An output force is created in a servomotor by the development of a pressure differential across a wall. The stepped diameter piston is connected to the wall. When the transfer control valve is activated, the output force is shifted from the first diameter to the second diameter to produce an added output potential. However, in the event that the pressure differential across the wall does not develop, the transfer control valve directly shifts the input force from the first diameter to the second diameter completely eliminating the development of the first pressure force.
In copending U.S. Application Ser. No. 484,381 and now U.S. Pat. No. 3,937,021, incorporated herein by reference, a hold-off means is disclosed for delaying the development of the second pressure force until the first pressure force reaches a predetermined valve before the transfer control valve is activated by the input force. In this servomotor means, a stepped piston means divides the pressurizing chamber of the pressurizing mechanism into a first section and a second section. The first section is connected to a relief chamber through the transfer control valve. The relief chamber is connected to a reservoir through a flow path. A tilt valve means located in the flow path allows communication of fluid from the reservoir, when a stem thereon engages the stepped piston, and prevents communication of fluid from the relief chamber when the stepped piston disengages the stem. As the stepped piston moves in response to the input force, the first piston transfers fluid from the first section through the transfer control valve into the relief chamber. The hold-off means prevents any fluid from escaping from the relief chamber until the first pressure force reaches a predetermined value. This same first pressure force is simultaneously supplied to the wheel brakes. When the first pressure force reaches the predetermined valve, the hold-off means releases fluid to the reservoir and permits the input force to act through that portion to provide an added output as a second pressure force for operating the wheel brakes. However, if the transfer valve is rapidly modulated, fluid from the second section will be transferred to the first section when the valve is released and afterwards released into the reservoir. This reduction in fluid from the brake system will allow the stepped piston means to contact the bottom of the bore in the second section before the optimum second pressure force is developed.
In copending U.S. Application Ser. No. 511,413 and now U.S. Pat. No. 3,910,046, incorporated herein by reference, a control means is disclosed for adding compensatory fluid to the braking system to replace the fluid transferred to the relief valve during the transfer of the input force from the first diameter to the second diameter of the power piston. However, in such a device if the shift of the operational mode from the first diameter to the second diameter in response to the input force is transmitted in an oscillating manner a corresponding change in the displacement volume of the power braking unit will occur which may result in an inadequate quantity of hydraulic fluid to operate the power braking system.
Later in copending U.S. Application Ser. No. 603,430 and now U.S. Pat. No. 3,972,191, incorporated herein by reference, a balanced control valve was added to provide the servomotor with a smooth transition when the operation was shifted from the first stage to the second stage.